It is desirable to know the travel time of blood from Pulmonary Arteries to the left atrium, for example, for delaying X-ray imaging start in an imaging procedure. The capability of delaying X-ray imaging facilitates accurate left atrium imaging (e.g. 3D imaging) of the left Atrium and capture of functional details. Known systems lack a flexible, user friendly capability of automatically calculating blood flow travel time between user selected anatomical sites. A system according to invention principles addresses this deficiency and related problems.